Starch is one of the materials most commonly used in paper sizing and paper coating compositions. It can be used, for example, as a thickener, emulsifier or binder. Depending on its role, the starch will have to have specific, pre-determined rheological properties. The desired properties can be achieved by a number of well known starch modification methods including thermal, chemical and enzymatic treatment.
One of the most commonly used starch derivatives is oxidised starches. Oxidised starches are commercially produced by treatment with agents such as sodium hypochlorite. There is a desire in the paper industry, however, to find a chlorine-free alternative to these starches. To this end, the use of starch derivatives, such as dextrins, has been proposed.
Dextrins can be produced by thermal, acid or enzyme modification, with their formation leading to a drop in the pH of the reaction medium. As a result, the use of dextrins in aqueous sizing and/or coating compositions must be accompanied by the use of a buffer. Sodium sulphite has been proposed for this role. It has been found, however, that sodium sulphite is easily oxidised upon contact with air and by oxidants present in water formulations. Compositions obtained in this manner are therefore unstable with pH fluctuations resulting in an undesirable browning effect.
A further difficulty with paper sizing and coating compositions is the presence of impurities and, in particular, the presence of bacterial and/or fungal contamination. These impurities may originate from the water used to form the aqueous composition or from other contaminated ingredients (e.g. pigments, fillers, etc.). The presence of micro-organisms can cause compositions to ferment leading to instability and reduced shelf-life. Fermentation can also result in an undesirable colouration of the composition and in the production of volatile gases (such as H2S) which have unpleasant odours.
A number of biocides have been suggested to tackle this problem. Unfortunately, the use of traditional biocides is rather complex. For instance, the type and number of spores present in a starch composition has to be determined before an appropriate biocide can be selected. The use of biocides therefore increases the level of technical expertise required for making up sizing and coating compositions. They cannot be pre-mixed to the initial starch composition as each user may require a different type of biocide in a different quantity. Thus, the use of traditional biocides adds to the amount of time necessary for the preparation of sizing and coating compositions and therefore raises the overall cost of the final product, effectively rendering the process uneconomical.
It is therefore apparent that an improved method of producing starch-based compositions for use in paper sizing and/or coating is required. The present invention provides such a method.